On the last show, I overheard something about the Earth’s magnetic poles reversing. Little explanation was given. I think I read elsewhere that this is a process that takes thousands of years. Yet you sometimes read that the reversal of the magnetic poles should take place about now because it takes place every few years. I find it confusing that they say it takes thousands of years and yet it happens every few years. How exactly does the reversal of the Earth’s magnetic poles take place? What is known about it?
Do you notice that on earth itself? Haven’t the poles been shifting since the beginning of our measurements?
Answer
There is no contradiction between “it takes thousands of years” and “it takes place every so many years,” since those “so many years” are much more than a thousand.
We know that the magnetic poles ‘reverse poles’ not only from the measurements we have made since we have measuring instruments. For earlier times we can deduce this from geological measurements. The Earth’s magnetic field magnetizes iron; where due to the movements of the earth’s crust liquid iron rises from the earth’s mantle to the surface, that iron is magnetized, and once it has solidified it maintains a permanent magnetic field. That magnetic field then indicates to this day how the strength and direction of the Earth’s magnetic field were when that magma came up. These polarity reversals are not strictly periodic; the timescales in question (those ‘so many years’ from just now) are typically on the order of a million years. The last polarity reversal has been longer than the time between the two previous ones, which is why it can be said that ‘it’s about time’.
Magnetic fields are created by moving charges. The Earth’s magnetic field is the result of large-scale movements deep within the Earth. We don’t have much control over that. It is likely that these movements sometimes stop (then the field decreases), and then on average start again in the other direction (the new magnetic field then has a reversed polarity). The shifting of the magnetic poles also has to do with the changed movements deep in the Earth.
Answered by
prof. Christopher Waelkens
Astronomy
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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